In recent years, there have been needs to increase the size and resolution of image display devices such as liquid crystal display devices, and the number of scanning signal lines and the number of video signal lines are increasing compared to the numbers used conventionally. As a result, it has become difficult to drive an image display device with one gate driver IC (an IC for driving the scanning signal lines) and one source driver IC (an IC for driving the video signal lines). Therefore, there have been provided image display devices including a plurality of gate driver ICs and a plurality of source driver ICs.
In some above-mentioned image display devices, the number of gradations that can be displayed by such an image display device may be less than the number of gradations in externally provided image data representing an image that is to be displayed (hereinafter, referred to as “original image data”). In such a case, fine differences between gradations in the image representing the original image cannot be represented, and therefore it is not possible to obtain satisfactory image quality. Accordingly, as known approaches for the pseudo multi-gradation display, ordered dithering and the error diffusion method have been used conventionally. According to the error diffusion method, an error between a gradation level represented by original image data and a gradation level actually used by an image display device for displaying the original image data is diffused from each pixel to its neighboring pixels. As a result, even if the number of gradations that can be displayed by the image display device is small, the pseudo multi-gradation display is provided to realize an image display with smoothly changing gradations. Note that a minimum unit of an image displayed on a screen is referred to herein as a “pixel”. In addition, an individual gate driver IC is referred to as a “gate driver unit (a scanning signal line drive unit)”, and an individual source driver IC is referred to as a “source driver unit (a video signal line drive unit)”. Furthermore, a circuit composed of a plurality of gate driver units or a plurality of source driver units and having a function of driving all scanning signal lines or video signal lines is referred to as a “gate driver (a scanning signal line drive circuit)” or a “source driver (a video signal line drive circuit)”.
Conversion of image data by the error diffusion method will be described below by taking as an example a case where original image data composed of pixel data in which each pixel is represented by eight bits is converted into pixel data in which each pixel is represented by one bit (hereinafter, the data resulted from the conversion is referred to as “display image data”). In this example, each piece of pixel data contained in the original image data is composed of eight bits, and therefore the number of gradations represented by the pixel data contained in the original image data is 256. On the other hand, each piece of pixel data contained in the display image data is composed of one bit, and therefore the number of gradations represented by the pixel data contained in the display image data is 2. For convenience of explanation, the gradation level represented by each piece of pixel data contained in the original image data is referred to as an “original image gradation level” and represented by 0 h to 255 h. In addition, the gradation level represented by each piece of pixel data contained in the display image data is referred to as a “display image gradation level” and represented by 0 k or 1 k.
Described first is conversion from the original image gradation level to the display image gradation level (hereinafter, referred to as “gradation conversion”). At the time of gradation conversion, the original image gradation level of each pixel is compared to a threshold (in this case, “128”). As a result, if the original image gradation level is 128 h or lower, the display image gradation level becomes 0 k. On the other hand, if the original image gradation level is 129 h or higher, the display image gradation level becomes 1 k. Here, supposing that an original image gradation level corresponding to a display image gradation level is represented by L(K), L(0 k) is 0 h, and L(1 k) is 255 h.
Next, referring to FIGS. 7A and B of FIG. 7B, an error caused by the gradation conversion is described. FIG. 7A is a representation for explaining an error in the case of an original image gradation level of 200 h. When the original image gradation level is 200 h, the display image gradation level is 1 k. In this case, L(K) is 255 h, and therefore a gradation level error corresponding to “200 h to 255 h” occurs as shown in part FIG. 7A. FIG. 7B is a representation for explaining an error in the case of an original image gradation level of 80 h. When the original image gradation level is 80 h, the display image gradation level is 0 k. In this case, L(K) is 0 h, and therefore a gradation level error corresponding to “80 h to 0 h” occurs as shown in FIG. 7B.
The error caused as described above is diffused from each pixel to predetermined pixels among its neighboring pixels at predetermined allocation rates. Such a process for diffusing the error is referred to as an “error diffusion process”, which is described below. FIG. 8 is a diagram for explaining the error diffusion process. In FIG. 8, the rectangle denoted by reference character Ga0 represents a pixel on a display screen (hereinafter, referred to as the “pixel of interest”), and the rectangles denoted by reference characters Ga1 to Ga4 represent right, lower left, underlying and lower right pixels, respectively, with respect to the pixel of interest. In addition, the coordinates of the pixel of interest Ga0 are represented by (i,j), and its original image gradation level is represented by f(i,j). In this case, an error Er(i,j) that occurs to the pixel of interest Ga0 at the time of gradation conversion is represented by the following equation (1).Er(i,j)=f(i,j)−L(K)  (1)Note that in the above equation (1), K is a display image gradation level of the pixel of interest Ga0 converted from the original image gradation level f(i,j).
The above-mentioned error Er(i,j) is diffused to the pixels Ga1 to Ga4 by the error diffusion process as shown in FIG. 8. The gradation levels of the pixels Ga1 to Ga4 after the error diffusion process are represented by the following equations (2) to (5), respectively.F(i+1,j)=f(i+1,j)+Er(i,j)×M1  (2)F(i−1,j+1)=f(i−1,j+1)+Er(i,j)×M2  (3)F(i,j+1)=f(i,j+1)+Er(i,j)×M3  (4)F(i+1,j+1)=f(i+1,j+1)+Er(i,j)×M4  (5)
In the above equations (2) to (5), M1 to M4 are coefficients representing the allocation rates for diffusing the error Er(i,j), which is caused to the pixel Ga0, to the pixels Ga1 to Ga4 (hereinafter, such coefficients are referred to as “diffusion coefficients”). For example, M1 to M4 are set to values such as 7/16, 3/16, 5/16 and 1/16.
The following description focuses on the point that errors of neighboring pixels are added to the original image gradation level of each pixel. FIG. 9 is a diagram for explaining addition of errors to the original image gradation level of a pixel. Looking at a pixel Ga10, errors caused to pixels Ga11 to Ga14 are added to the original image gradation level of the pixel Ga10 as shown in FIG. 9. Here, when a value obtained by multiplying the error caused to the pixel Ga11 by a diffusion coefficient for diffusing the error to the pixel Ga10 is represented by er11 (similar for the pixels Ga12 to Ga14), a gradation level of the pixel Ga10 after the addition of the errors is represented by the following equation (6). Note that f is an original image gradation level of the pixel Ga10.F=f+er11+er12+er13+er14  (6)
For all pixels, the error diffusion process is performed in parallel to the gradation conversion as described above.
FIG. 10 is a diagram schematically illustrating an example of a display screen, which is provided in the case where the above-described error diffusion process is applied to an image display device having a source driver composed of a plurality of source driver units. The image display device includes three source driver units 301, 302 and 303, which drive video signal lines to display an image on their respective regions each corresponding to one third of the entire display screen (hereinafter, each region is referred to as a “display block”). In this case, vertically running lines as denoted by reference character Z in FIG. 10 (hereinafter, referred to as “vertical stripes”) are visually recognized at boundaries of display blocks. This is described with reference to FIG. 11.
FIG. 11 is a diagram for explaining the error diffusion process at a boundary of a display block. Looking at the pixel Ga10 in FIG. 11, the gradation level F of the pixel Ga10 after addition of errors should be a value as represented by the above equation (6). However, at the boundary of the display block, the gradation level F of the pixel Ga10 cannot be added with the error er11 of the pixel Ga11 and the error er14 of the pixel Ga14. Thus, at the boundary of the display block, the errors caused at the time of gradation conversion are not diffused beyond the boundary, and therefore vertical stripes are visually recognized.
Japanese Laid-Open Patent Publication No. 5-328265 discloses a liquid crystal display device in which the vertical stripe is suppressed from being visually recognized on a display screen, the vertical stripe caused by that a source driver is composed of a plurality of source driver units. In this liquid crystal display device, the difference in sampling voltage between the source driver units is corrected by changing a source voltage value of a drive unit, and therefore any vertical stripe as described above is suppressed from occurring.
[Patent Publication 1] Japanese Laid-Open Patent Publication No. 5-328265